Described herein are adhesive composites for use with biomedical electrodes. The adhesive composites are of the pressure sensitive type, being adherent to a person's skin by pressing the composite onto the skin. Application of the composite to the skin effectuates an electric circuit between the skin and medical electrostimulating or recording devices that communicate via suitable attachments with the composite.
By way of background, it is important to understand various features of adhesive composites that are employed with biomedical electrodes. Generally they contain an electrically conductive paste or gel and a pressure sensitive component. To some extent the paste or gels have adhesive properties, but that is not their primary function; rather they primarily serve as electrical conductors. In contrast, the adhesive component acts to affix the composite to a suitable surface and is functionally related to adhesive tapes well known to those in the art. It is important to note though, that the function of the adhesive composite, unlike adhesive tapes, is twofold; first, it acts to secure the biomedical electrode firmly to the patient's skin, and, second, it provides a current path between the electrode itself and the surface of the skin. A further property of the adhesive composite is that it also must adhere to a backing material in those instances where the same is employed with the electrode device.
Adhesive composites are a subclass of adhesive materials used for a variety of purposes and that can be categorized into one of three classes, depending upon whether the adhesive property is initiated by pressure, heat, or is solvent activated. Solvent activated tapes are in every day use, a common example being "gummed tapes" used for sealing cartons, envelopes and the like. Their adhesive properties are realized by applying a solvent, usually water to the tape, and then applying the tape to a suitable surface.
Adhesives that are activated by heat are most often employed in garment repair, fabric mending, or the like. The adhesive nature of these materials is a result of a bond produced by heat between the adhesive material and the substrate material to which it is applied.
The third class of adhesive materials consists of those that are activated by pressure and are termed "pressure sensitive" composites. The adhesive composite of the subject patent would be considered a member of this class. A key characteristic of these adhesives is that they adhere rapidly to most solid surfaces as a result of moderate finger pressure. Such adhesives can be readily and easily secured to the skin to provide a current path between the skin and the electroconductive elements of a biomedical electrode. Further desirable properties of such adhesives are that they can be easily removed from a patient's skin without having to employ excessive force or solvents.
The prior art shows a number of adhesive composites used with biomedical electrodes. In order to appreciate the technical problems associated with developing suitable adhesive composites, it is important to note that they were initially developed to avoid using adhesive paste, gels and the like, which served a dual purpose of enhancing conductivity of the electrode with the skin, as well as securing the electrode to the skin. These types of electrodes are referred to as "wet" electrodes. To avoid the sloppiness and packaging problems associated with "wet" electrodes, adhesive composites were developed that are termed "dry" electrodes. Although "dry" electrodes have circumvented the problems associated with "wet" electrodes, they generally do not have high electrical conductivity with the skin primarily because of the presence of conductive filler needed to impart to the composite the necessary adhesive and cohesive properties desired. Thus, often "dry" electrodes have a high signal-to-noise ratio. This problem has remained largely unsolved.
The prior art shows a number of patents directed to "dry" electrodes. U.S. Pat. No. 4,125,110 shows a naturally occuring polymer, gum karaya, used for securing the electrode to the skin. Gum karaya is a polysaccharide containing bound metallic ions. In contrast to the use of natural polymers, U.S. Pat. Nos. 4,273,135 and 4,352,359 show several types of acrylic based polymers employed as the conductive element in a "dry" disposable biomedical electrode adhesive composite. Further, in addition to acrylic or methacrylic polymers, U.S. Pat. No. 4,248,247, and abandoned U.S. patent application Ser. No. 049,212 filed June 21, 1979 show the use of polymers of isobutylene and butylenes.
In addition to the patents referred to above, other patents describe the use of filler materials having both high electrical conductivity and adhesive properties. For instance U.S. Pat. No. 4,524,087 describes an electrically conductive material consisting of polymers of polyhydric alcohol. The use of the latter compound is thought to enhance the conductive properties of the adhesive composite. Further, U.S. Pat. Nos. 4,588,762 describes a pressure sensitive adhesive composite for use with disposable biomedical electrodes wherein the composite consists of a viscoelastic polymeric adhesive phase and a electrically conductive aqueous phase containing a water receptive polymer, a humectant and an electrolyte. The electrically conductivity properties of the composite are enhanced by intimately dispersing the two phases.
Considerable effort has gone into maximizing the electrically conductive properties of adhesive composites used to fabricate biomedical electrodes, with little or no compromise in the adhesive property of the composite. At present there is a need for adhesive composite formulations that are suitably adherent to the skin, yet are skin conformable and have high electrical conductivity.